Air heater system



NW. 15 194% .5. CASSIE 2,488,267

AIR HEATER sysma 2 Shee'ts-Sheet 1 Filed Aug. 30, 1947 IN WIN TOR JEhnCass ie WJM Nov. 15, 1949 J. CASSYIE 2,488,267

AIR HEATER SYSTEM Filed Aug. 30, 1947 2 Shams-Sheet 2 INVEN TOR \7'o hr;Cassie Patented Nov. 15, 1949 AIR HEATER SYSTEIVI John Cassie, HydePark, Masa, assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application August 30,1947, Serial No. 771,467 Claims. (01. 257-244) This invention relates tosystems for the extraction of heat from the flue gases of steam plants,and the heating of air for combustion with the extracted heat.

In steam power plants it is usual to connect air heaters to the outletsof forced draft fans,

flue gases being passed over the exteriors of the v air heater tubes,and the fans blowing the air for combustion, through the. tubes. At lowboiler loads, the upstream header sheet of such an air heater, is oftenbelow the dew point temperature of corrosive vapors in the flue gases,with the result that acid condenses on the sheet and corrode same. Forpreventing this, it has been the practice to recirculate heated air fromthe downstream end of the air heaters into the inlets of theirassociated fans, thereby recirculating previously heated air through theair heaters and decreasing the volume of cold air passed therethrough.Without inlet boxes on the fans, the outlet ends of the recirculationducts have to be close to the inlets of the fans for preventing cold airinstead of the recirculated, heated air, from entering the fans betweenthe outlets of the recirculation ducts and the fan inlets. Such ductsinterfer with the natural flow into the fan inlets when no recirculationis required, thus reducing the capacities and efficiencies of the fans.If an inlet box is used for each fan inlet, the capacity and efliciencyof the fan may not be afiected but the cost of the fan is greatlyincreased since a longer fan shaft and larger bearings are required.

Another method of recirculating the heated air Without affecting theperformance of a forced draft fan, is to provide a separate fan forrecirculating the heated air. This, of course, adds to the cost of thesystem.

This invention overcomes the disadvantages described in the foregoing byproviding the outlet end of a recirculation duct with an extensionshaped to discharge the recirculated air efliciente ly into the faninlet for which it is provided, the

I duct extension being retractable so that when recirculation of theheated air is not required, it can be withdrawn from the fan inlet so asnot to interfere with the natural flow of air thereinto.

Objects of the invention are to reduce the cost of, and to improve theperformance of, air heater systems for steam power plants.

The invention will now be described with reference to the drawings, ofwhich:

Fig. l. is a side elevation, partially in section, of an air heatingsystem embodying this invention, the outlet end of the recirculationducts be- 2. ing extended for supplying the fan inlets;

Fig. 2 is a sectional view along the lines 22 of. Fig. 1;

Fig. 3 is a view similar to Fig. 1 except that the outlet ends of therecirculation ducts are retracted from the fan inlets, and

Fig. 4 is a sectional view along the lines 4-4 of Fig. 3.

The forced draft fan III has an air inlet II on each side of its wheel[2, and has its outlet l3 connected through the transformation duct I4to the inlet of the air heater IS, the outlet of which supplies heatedair through the duct IT to the combustion chamber of a boiler. Forsimplifying the drawing, the motor for driving the fan, and the journalsfor the fan shaft are not shown.

The air heater l5 contains the plurality of tubes l8 through theinteriors of which the air from the fan is blown. The duct I9 suppliesflue gas from the boiler over the exteriors of the tubes I8 for heatingthe air passing therethrough, this construction being conventional.

The recirculation ducts 20 have their inlets 2! connected into theinterior of the heated air duct l1, and have their outlet ends 22 formedaround the retractable ducts 23. The ducts 23 have the air outlets 24having their upper and. lower sides formed in circular arcs thecentersof which are recirculated air into aligned with the centers of the airinlets H of the fan, and the radii of which are such that the outlets 24extend partially around the fan inlets concentrically in alignmenttherewith when the ducts 23 are fully extended as illustrated by Figs. 1and 2.

The ducts 20 contain the dampers 25 which are adapted to be adjusted byan attendant for regulating the volume of air recirculated through thefan, and contain the dampers 26 which contact the inturned upper ends ofthe ducts 23, and which are adapted to be closed by the ducts 23 whenthey are retracted as illustrated by Figs. 3 and 4.

The retractable ducts 23 are adapted to be retracted to the positionsshown by Figs. 3 and 4 when no recirculation of air through the airheater is required, and are adapted to be extended to the positionsshown by Figs. 1 and 2 when recirculation is required. This retractionand extension of the ducts 23 would in the embodiment illustrated, beaccomplished manually by an attendant.

When the steam load of the associated boiler or boilers, is so low thatcondensation of acid from the flue gas might condense on the tubes wouldbe extended to the positions shown by Figs. 1 and 2. At such times theinturned upper ends of the ducts 23 would move downwardly sufficientdistances to permit the inner ends of the pivoted dampers 26 to fall -bygravity thus opening these dampers. The attendant would then set thedampers 25 to provide the volume of recirculated air which experience orinstruments would indicate as being necessary for the reduced boilerload. With the ducts 23 extended, their outlets are in such closealignment with the fan inlets that the heated air therethrough is drawndirectly into the fan inlets without escaping into the apparatus room,and that no room air is drawn into the fan inlets in the spaces betweenthe outlets 24 and the fan inlets. The increased volume of heated airthrough the heater tubes will raise the temperature of their outersurfaces above the dew point temperature of the corrosive vapors in theflue gases.

When the plant load has risen to the point that no recirculation ofheated air is required, the attendant will retract the ducts 23 to thepositions shown by Figs. 3 and 4. When this is done the inturned upperends of the ducts 23 will move the dampers 26 to closed positions thusshutting off the flow of air through the ducts 20 and 23, regardless ofthe positions of the dampers 25. With the ducts 23 fully retracted,their outlets 24 are sufficiently removed from the inlets II of the fanthat they do not interfere with the air flow thereinto of air from theapparatus room.

While one embodiment of the invention has been described for the purposeof illustration, it should be understood that the invention is notlimited to the exact apparatus and arrangement of apparatus illustrated,as modifications thereof may be suggested by those skilled in the art,without departure from the essence of the invention.

What is claimed is:

1. In an air heating system including an air heater and a fan for movingair through the heater, a recirculation duct connecting the outlet ofthe heater with the inlet of the fan for recirculating air from theheater therethrough, dampering means in said duct, said duct having anoutlet adjustable in one position to overlap the inlet of the fan, andadjustable in the opposite position so as to be substantially out of thepath of the air entering the fan, and means for opening said damperingmeans when said outlet is moved to said one position, and for closingsaid dampering means when the outlet is moved to the opposite position.A

2. In an air heating system including an air heater and a fan for movingair through the heater, the fan having a circular, axial inlet, arecirculation ductconnecting the outlet of the heater with the inlet ofthe fan for recirculating air through the heater, said duct having anoutlet adjustable in one position to overlapsaid inlet, and adjustablein the opposite position so as tobe substantially .out of the path ofthe air entering said inlet, said duct outlet having inner and outerportions curved in concentric circular arcs which when said outlet ofsaid duct is adjusted to said one position, are concentric with said faninlet.

3. The invention claimed in claim 2 in which dampering means in the ductis opened by movement of the duct outlet to the one position, and isclosed by movement of the duct outlet to the opposite position.

4. In an air heating system including an air heater, a fan formoving airthrough the heater, and a recirculation duct for recirculating airthrough the heater, said fan having a circular, axial inlet, thecombination of an outlet duct slidable in the recirculation duct to oneposition where the outlet thereof overlaps said 'fan inlet, and slidableto the opposite position where its outlet is substantially removed fromthe path of l the airentering the fan inlet, said outlet duct havinginner and outer portions forming said outlet which are curved incircular arcs concentric with said fan inlet.

5. The invention as claimed in claim 4 in which dampering means in therecirculation duct is opened by movement of the outlet duct to the oneposition, and is closed by movement of the outlet duct to the oppositeposition.

' JOHN CASSIE.

REFERENCES CITED FOREIGN PATENTS Country Date Great Britain Oct. 16,1933 Number

